1. Field of the Invention
The present invention relates to a switching power supply circuit, and particularly to a switching power supply circuit including a cascode element.
2. Description of the Background Art
Electrical appliances such as a television, a refrigerator and a washing machine include a power supply circuit that rectifies an AC voltage to generate a stable DC voltage or converts a DC voltage into a different DC voltage. The power supply circuit is sometimes called “converter” because of its conversion function. There are many types of the power supply circuit, such as a forward converter, a flyback converter, a boosting converter, and a stepping-down converter. These are appropriately used depending on an AC voltage, a DC voltage, a combination of voltages to be converted, and power to be handled.
Japanese Patent Laying-Open No. 2004-194372 discloses a boosting converter circuit that stores power in an output capacitor from a primary power supply through an inductor and a diode, thereby generating a boosted DC output voltage. A control circuit outputting a control signal in response to the DC output voltage controls a gate terminal of a power MOSFET controlling a current of the inductor, thereby keeping the output voltage of the boosting converter circuit constant.
Japanese Patent Laying-Open No. 2010-130881 discloses a switching power supply circuit including a PFC (Power Factor Control) circuit. A choke coil and a diode are serially connected between an output terminal of a rectification circuit and one terminal on the primary winding side of a transformer, and the PFC circuit controls a current flowing through the choke coil. The PFC circuit includes a switching element and a resistor serially connected between one terminal of an output of the rectification circuit and a point connecting the choke coil and the diode. ON/OFF of a switching element is controlled based on a potential of a point connecting the switching element and the resistor as well as an output of a flip flop circuit to which a voltage of the primary winding side of the transformer is inputted.
Japanese Patent Laying-Open No. 2011-142265 discloses a cascode-connected circuit including a normally-on type transistor and a normally-off type transistor. The normally-on type transistor is a nitride semiconductor transistor and the normally-off type transistor is a silicon semiconductor transistor. Such a configuration allows the nitride semiconductor transistor to be operated as normally off.
In the conventional switching power supply circuit, the potential of the point connecting the switching element and the resistor is fed back to the switching element, thereby controlling the current flowing through the choke coil. Since the current flowing through the choke coil flows to the resistor as well via the switching element, heat loss occurs at the resistor. This heat loss is responsible for decrease in reliability of the entire switching power supply circuit. For example, when a current of 4 Arms constantly flows through a resistor having a resistance value of 0.2Ω, heat loss caused by the resistor is 3.2 W.
In addition, the presence of such a heat generating component (resistor) makes heat release design of the entire switching power supply circuit difficult. This is because the presence of a component like an electrolytic capacitor, whose failure rate increases as the temperature increases, leads to decrease in life of the entire switching power supply circuit.